1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and a charged particle beam writing method. For example, the present invention relates to a writing apparatus that writes or draws a pattern on a substrate to which a substrate cover has been attached, by using an electron beam, and to a method thereof.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. In order to form a desired circuit pattern on semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high precision is needed. Thus, the electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 19 is a schematic diagram explaining operations of a variable shaped electron beam writing apparatus. As shown in the figure, the variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g. X direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the X direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam method.
In a writing apparatus, there is a case of performing writing while covering the periphery of a mask (substrate) with a frame-like mask cover so that the insulated portion at the end surface of the mask serving as a target object may not be charged by reflected electrons of electron beams irradiating the mask. In such a case, the mask cover is kept in the transfer channel in the vacuum of the writing apparatus, and placed on the mask during the writing. When placing the cover on the mask, minute positional deviation may occur, for example, because the contact portion between the mask and the mask cover does not contact simultaneously. Moreover, positional deviation may occur, for example, because of a positioning error of the attaching/detaching mechanism at the time of placing the mask cover on the mask. Although positional deviation of the mask cover generated by one transferring operation or one writing operation is minute, if the cover is used for writing a plurality of times without performing alignment, unacceptable positional deviation may occur due to accumulated errors. Therefore, in order not to accumulate such errors, it is desirable to perform alignment each time before placing the cover on the mask so that the cover may be placed at a highly accurate position with respect to the mask. Thus, conventionally, an alignment function is provided in the support table which supports the mask cover, and mask cover alignment is performed every time the mask cover is supported. However, in such a supporting mechanism, when detaching the mask cover from the mask, since an alignment operation starts at the time of the mask cover having been supported by the supporting mechanism, there is a problem in that if the position is shifted, the mask cover slides in the direction parallel to the mask surface in order to correct the position, thereby scratching the mask. Accordingly, there is proposed a method of separately providing a support table for supporting the mask cover and a support table for performing alignment of the mask cover, and of executing alignment of the mask cover after detaching the mask cover from the mask (refer to, e.g., Japanese Patent Application Laid-open 2011-14630).
However, according to this mechanism, the portion in contact with the mask cover becomes increased because the supporting surface of the support table for performing mask cover alignment is added to the supporting surface of the support table for supporting the mask cover. Therefore, since the contact portion increases, the risk of generation of particles at the contact portion increases. That is, there is a problem that when the risk of particle generation increases, the risk of the particles coming onto the mask also increases.